Since Facebook bought Instagram in April, it's been apparent that Facebook and Twitter, which rank one-two in popularity among social-networking users, are distancing themselves from each other.

In the Internet age, data and dwell time equal money. With Facebook pushing to grow its mobile revenue (and modest stock price) and Twitter still searching for effective ways to translate its popularity into profit, this rift was perhaps inevitable.

In July, Twitter stopped letting Instagram users find friends via Twitter they may want to follow on the photo app. (They did the same for popular blogging site Tumblr after buying rival blogging platform Posterous.)

In the most recent move, users began noticing that images posted to Twitter were cropped weirdly. By Monday, sending an Instagram photo to Twitter simply posted a link directing followers to Instagram's recently beefed-up website.

"We're working on building an awesome Web presence, which we just launched," Systrom said. "We revamped our Web properties, and now we're able to staff up teams to work on Web properties with the Facebook acquisition."

A Facebook spokeswoman confirmed Monday that Instagram turned off support for "Twitter cards," the app that lets third-party images appear on the site. In its statement posted Sunday, Twitter also confirmed what happened.

"Instagram has disabled photo integration with Twitter. As a result, photos are no longer appearing in Tweets or user photo galleries," the statement reads. "While tweeting links to Instagram photos is still possible, you can no longer view the photos on Twitter, as was previously the case."

At Le Web, Systrom said the shift wasn't payback for Twitter shutting down its friend-finder function. But he also said there are no plans to disable Instagram images on other sites.

"This is more of a one-off," he said.

Underlying all the back-and-forth, of course, is the possibility that hard feelings still exist after Twitter's reported offer to buy Instagram was spurned in favor of a reported $1 billion deal with Mark Zuckerberg's Web juggernaut.

By allowing its images to show up on Twitter, Instagram gave Twitter users no incentive to visit its own site or mobile app. The amount of time visitors stay on a website is an important figure for advertisers choosing which sites to patronize. Keeping Instagram photos off Twitter also could encourage users to publish their pictures to Facebook, which allows them to show up in all their glory.

Meanwhile, there are reports that Twitter is planning its own photo-filtering app, which could be out by the end of the year.

Such moves and countermoves are to be expected, many in the tech blogosphere were saying Monday.

"The companies obviously realize how important photos are to getting users to share and interact on the web, so it looks like the competition isn't stopping any time soon," wrote Eliza Kern for GigaOM.

Others were saying this won't change the actual Twitter-Instagram user experience all that much.